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Understanding the characteristics of drawing body shape is essential for optimization of drawing parameters in longwall top coal caving mining. In this study, both physical experiments and theoretical analysis are employed to investigate these characteristics and derive a theoretical equation for the drawing body shape along the working face in an inclined seam. By analyzing the initial positions of drawn marked particles, the characteristics of the drawing body shape for different seam dip angles are obtained. It is shown that the drawing body of the top coal exhibits a shape-difference and volume-symmetry characteristic, on taking a vertical line through the center of support opening as the axis of symmetry, the shapes of the drawing body on the two sides of this axis are clearly different, but their volumes are equal. By establishing theoretical models of the drawing body in the initial drawing stage and the normal drawing stage, a theoretical equation for the drawing body in an inclined seam is proposed, which can accurately describe the characteristics of the drawing body shape. The shape characteristics and volume symmetry of the drawing body are further analyzed by comparing the results of theoretical calculations and numerical simulations. It is shown that one side of the drawing body is divided into two parts by an inflection point, with the lower part being a variation development area. This variation development area increases gradually with increasing seam dip angle, resulting in an asymmetry of the drawing body shape. However, the volume symmetry coefficient fluctuates around 1 for all values of the seam dip angle variation, and the volumes of the drawing body on the two sides are more or less equal as the variation development volume is more or less equal to the cut volume. Both theoretical calculations and numerical simulations confirm that the drawing body of the top coal exhibits the shape-difference and volume-symmetry characteristic.

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This review details the state of the art in research on top coal drawing mechanisms in Longwall top coal caving (LTCC) by examining the relevant literature over the last two decades. It starts with an introduction of the brief history and basic procedures of LTCC. The framework of research on the drawing mechanism, basic concepts, and some theoretical models of LTCC are detailed in sect. research framework of top coal drawing mechanism. The authors note that the Top coal drawbody (TCD), Top coal boundary (TCB) and Top coal recovery ratio (TCRR) are key factors in the drawing mechanism. The Body–boundary–ratio (BBR) research system has been the classic framework for research over the last 20 years. The modified Bergmark–Roos model, which considers the effects of the supporting rear canopy, flowing velocity of top coal, and its shape factor, is optimal for characterizing the TCD. A 3D model to describe the TCB that considers the thicknesses of the coal seam and roof strata is reviewed. In sect. physical testing and numerical simulation, the physical tests and numerical simulations in the literature are classified for ease of bibliographical review, and classic conclusions regarding the drawing mechanism of top coal are presented and discussed with elaborate illustrations and descriptions. The deflection of the TCD is noted, and is caused by the shape of the rear canopy. The inclined coal seam always induces a larger TCD, and a deflection in the TCD has also been observed in it. The effects of the drawing sequence and drawing interval on the TCRR are reviewed, where a long drawing interval is found to lead to significant loss of top coal. Its flowing behavior and velocity distribution are also presented. Sect. practical applications of drawing mechanisms for LTCC mines 4 summarizes over 10 cases where the TCRR of LTCC mines improved due to the guidance of the drawing mechanism. The final section provides a summary of the work here and some open questions. Prospective investigations are highlighted to give researchers guidance on promising issues in future research on LTCC.